I have two parts available on Shapeways for turning Atlas U23B models into reasonable facsimiles of a U36B or U33B. The first solution requires cutting off part of the U23B radiator and fitting a whole new flared radiator over the cut section.
The second solution simply requires gluing flared radiator wings onto the sides of the U23B radiator.
I use method two for my own models just because it is very easy and can easily be applied to a factory painted model. I paint the flared radiator wings the proper color. And I attach the flared radiator wings with Elmer’s white glue so I can wet them and remove them if I ever desire to do so.
If you aren’t satisfied with the second solution due to the existing U23B radiator, which is too narrow and doesn’t extend to the flared section, I provide a choice of two vector images (white background and gray) that can be printed as a decal or on paper to represent the radiator screen and to cover the existing U23B radiator. To use the radiator graphics, scrape the grab iron on the top of the existing radiator off with a razor blade. Print the screens (ideally using a laser printer), glue the screens down with Elmer’s glue if using paper. You could print the graphic with the white background on colored paper. Or you could use the dirty gray screen graphic.
Get the parts at Shapeways here and here. As always, with parts in the Frosted Detail materials, don’t skip the step of using a soak in Bestine (or Doc Edington’s brand) to properly clean the parts of support wax.